Promoter and Riboswitch Control of the Mg2 Transporter

The MgtA protein from Salmonella enterica serovar Typhimurium mediates Mg2 uptake from the periplasm into the cytoplasm. Here we report that the PhoP/PhoQ two-component regulatory system, which responds to periplasmic Mg2, governs mgtA transcription initiation at all investigated Mg2 concentrations and that the Mg2-sensing 5 leader region of the mgtA gene controls transcription elongation into the mgtA coding region when Salmonella is grown in media with <50 M Mg2. Overexpression of the Mg2 transporter CorA, which is believed to increase cytoplasmic Mg2 levels, decreased mgtA transcription in a manner dependent on a functional mgtA 5 leader. Expression of the Mg2 transporter gene mgtA from Salmo-nella enterica serovar Typhimurium is regulated at both the transcription initiation and elongation steps. Transcription ini-tiation is dependent on the two-component regulatory system PhoP/PhoQ (9), which is activated in response to low Mg2 (8), acid pH (16), and antimicrobial peptides (1) sensed in th